Now I don't know what "rules" you know or how formal you have to be, but you should be able to see that (A'+B) = B given A+B = 1. Clearly, if A'+B = 0, then B = 0. If A'+B = 1, we want B to also necessarily be 1. But suppose it's not, so suppose A'+B=1 but B=0. Then A' = 1, which means A=0. So both A and B would be 0, but we're given A+B = 1, which is a contradiction, so B does indeed have to be 1 if (A'+B) is 1.